1. Field of the Invention
The present invention relates to a current detection circuit which detects a current flowing through the current path in a motor driving device.
2. Description of the Related Art
In a motor driving device which drives motors within a machine tool, an industrial machine, a forging machine, an injection molding machine, or various robots, AC power input from an AC power supply side is temporarily converted by a converter into DC power, which is further converted into AC power by an inverter. The AC power is used as driving power for the motors.
FIG. 4 is a circuit diagram illustrating a general motor driving device. In, e.g., a motor driving device 100 which drives a three-phase AC motor 200, the DC input side of an inverter 51 is applied with a DC voltage from a DC power supply and outputs a three-phase AC current for driving the motor 200. A smoothing capacitor 53 is located on the DC input side of the inverter 51. Although not particularly illustrated herein, a converter (rectifier) which converts an AC current input from a commercial AC power supply into a DC current and outputs the DC current is generally located on the DC input side of the inverter 51.
The inverter 51 is implemented in a full-bridge inverter including upper and lower arms, each of which is provided with switching elements Su1, Sv1, Sw1, Su2, Sv2, and Sw2 including an inverse parallel circuit consisting of reflux diodes. More specifically, a series circuit is formed by the switching elements Su1 and Su2 for phase u, another series circuit is formed by the switching elements Sv1 and Sv2 for phase v, and still another series circuit is formed by the switching elements Sw1 and Sw2 for phase w. Gate driving commands Gu1, Gv1, Gw1, Gu2, Gv2, and Gw2 are supplied from a current control unit 52 to the gates of the switching elements Su1, Sv1, Sw1, Su2, Sv2, and Sw2, respectively, and used for ON/OFF control of the switching elements Su1, Sv1, Sw1, Su2, Sv2 and Sw2, respectively. With this operation, the inverter 51 converts DC power input from the DC input side into an AC current having desired frequencies and voltages for driving a three-phase AC motor.
The current control unit 52 generates gate driving commands Gu1, Gv1, Gw1, Gu2, Gv2, and Gw2, based on an input current command and a feedback value for an AC current flowing from the inverter 51 into the motor 200. The AC current flowing from the inverter 51 into the motor 200 is detected by a current detection circuit 1001. To generate appropriate gate driving commands Gu1, Gv1, Gw1, Gu2, Gv2, and Gw2 to accurately control the motor 200, it is important to perform the above-mentioned current feedback control using a detected current value detected by the current detection circuit 1001 with high accuracy.
One current detection method for the current detection circuit uses a shunt resistance scheme in which a resistor (shunt resistor) is inserted into a current path for current detection, and a voltage generated across the two ends of the resistor when a current flows through the resistor is detected to obtain a current value based on this voltage, as disclosed in, e.g., Japanese Unexamined Patent Publication No. 2014-14252. A shunt resistor is used to convert a current value in the shunt resistor into a voltage across the two ends of the shunt resistor, and the voltage across these two ends is transmitted to an arithmetic circuit in the form of an analog differential signal. However, due to the adverse effect of external noise in the transmission line, the analog differential signal varies, thus lowering the current detection accuracy. Examples of the external noise may include electromagnetic induction. In electromagnetic induction, a magnetic flux generated upon fluctuations in current within a given current path causes differences in potential around the current path. The larger the amount of variation in current flowing through a current path acting as a source of noise or the smaller the distance from a current path acting as a source of noise, the higher the level of noise generated in the current path due to electromagnetic induction. Since, especially, a motor driving device includes current paths suffering considerable variations in current which lead to noise due to electromagnetic induction, a current detection circuit which detects a current flowing through a motor is often disposed in proximity to each such current path. It is, therefore, important to design a current detection circuit which detects a current flowing through a motor, free of the influence of noise due to electromagnetic induction generated in such current paths.
In a current detection circuit of the shunt resistance scheme, one method for reducing the adverse effect of external noise is used to dispose differential signal lines in proximity to each other. FIG. 5 is a circuit diagram for explaining a general method for reducing the adverse effect of external noise in a current detection circuit of the conventional shunt resistance scheme. A current detection circuit 1001 of the general shunt resistance scheme includes a shunt resistor 61, a difference operation unit 63, and transmission lines 62-1 and 62-2. The shunt resistor 61 is placed in a current path for current detection. The difference operation unit 63 includes differential input and output terminals on the current input and output sides, respectively, of the shunt resistor 61. The transmission lines 62-1 and 62-2 connect the current input and output terminals of the shunt resistor 61 to the positive and negative input terminals, respectively, of the difference operation unit 63. When the transmission lines 62-1 and 62-2 are disposed in proximity to each other, external noise generated from an external noise source 300 similarly adversely affects the transmission lines 62-1 and 62-2 (referring to FIG. 5, reference numeral 301 denotes the waveform of noise components). However, signals respectively transmitted via the transmission lines 62-1 and 62-2 are input to the positive input terminal (+) and the negative input terminal (−), respectively, of the difference operation unit 63, and noise components of each signal are canceled by difference operation by the difference operation unit 63 so that the adverse effect of common-mode noise disappears. Referring to FIG. 5, reference numeral 401 denotes the waveform of a signal output from the difference operation unit 63.
FIG. 6 is a circuit diagram for explaining the adverse effect of external noise when transmission lines which connect the current input and output terminals of a shunt resistor to the positive and negative input terminals, respectively, of a difference operation unit are disposed without proximity to each other in a current detection circuit of the conventional shunt resistance scheme. When a transmission line 62-1 which connects the current input terminal of a shunt resistor 61 to the positive input terminal (+) of the difference operation unit 63 and a transmission line 62-2 which connects the current output terminal of the shunt resistor 61 to the negative input terminal (+) of the difference operation unit 63 are disposed without proximity to each other, a difference occurs in adverse effect of external noise between the transmission lines 62-1 and 62-2 (referring to FIG. 6, reference numeral 302 denotes the waveform of noise components). Noise components of signals respectively transmitted via the transmission lines 62-1 and 62-2 may not be canceled, depending on how difference operation is performed by the difference operation unit 63, and the adverse effect of external noise may therefore remain in a signal output from the difference operation unit 63. Referring to FIG. 6, reference numeral 402 denotes the waveform of a signal output from the difference operation unit 63.
As described above, in a current detection circuit of the shunt resistance scheme, to reduce the adverse effect of external noise, transmission lines which connect the current input and output terminals of a shunt resistor to the positive and negative input terminals, respectively, of a difference operation unit are desirably disposed in proximity to each other.
Unfortunately, since it is difficult in practice to dispose these transmission lines in proximity to each other. Especially transmission lines in the vicinity of the shunt resistor and transmission lines in the vicinity of the difference operation unit may not be disposed in proximity to each other due to factors associated with the physical structures of the shunt resistor and the difference operation unit. Therefore, noise components of signals respectively transmitted via the transmission lines may not be canceled, depending on how difference operation is performed by the difference operation unit, and the adverse effect of external noise may therefore remain in a signal output from the difference operation unit. Especially a motor driving device includes a large number of current paths which pass high currents so noise due to electromagnetic induction has a remarkable adverse effect. When a detected current value detected by a current detection circuit includes the adverse effect of external noise, a motor driving device which uses the detected current value for feedback control may not generate appropriate gate driving commands and, in turn, may not achieve precise motor control.